1. Field of the Invention
The invention relates to a single mode optical fiber which may be used for transmitting radiation at substantially higher powers than may be achieved using conventional means. The fiber does not suffer from non linear effects or optical damage at high powers to the same extent as conventional optical fibres. In particular, the fiber may be used as a single mode waveguide, in a single mode fiber laser or in a single mode fibre amplifier.
2. Description of the Related Art
Optical fibres are widely used to deliver light from one point to another and have application in communications, imaging and sensing. Conventionally, a typical optical fibre is a long strand of transparent material which is uniform along its length but which has a refractive index varying across its cross-section. For example, a central core region of higher refractive index is surrounded by a cladding region with a lower refractive index. Such a fibre may be made from fused silica with a cladding of pure silica surrounding a core made from silica into which deliberate impurities have been introduced to raise the refractive index. Light is confined in or near the core by the process of total internal reflection which takes place at the boundary between the core and the cladding.
In general, a fibre of this type may support more than one guided mode of propagation confined to the core (i.e. multi mode fibre), these modes travelling along the fibre at different phase velocities. However, if the core is made to be sufficiently small, only one guided mode of propagation will be confined to the core, the fundamental mode (i.e. a single mode fibre). That is, the distribution of light emerging from the fibre is unchanged when the conditions at the launch end of the fibre are changed and when the fibre itself is subjected to disturbances such as transverse compression or bending. Typically, a fibre designed to carry single mode light having a wavelength of 1500 nm may have a few percent of germanium dopant in the core, with a core diameter of 9 .mu.m.
More recently, a photonic crystal fibre (PCF) has been developed comprising a cladding made of a transparent material in which an array of holes are embedded along the length of the fibre [J. C. Knight, et al., Opt. Lett. 21 (1996) p. 1547. Errata: Opt. Lett. 22 (1997) p. 484]. The holes are arranged transversely in a periodic array and are filled with a material which has a lower refractive index than the rest of the cladding, the core of the fibre comprising a transparent region which breaks the periodicity of the cladding. Typically, both the core and the cladding are made from pure fused silica and the holes are filled with air. The core diameter is approximately 5 .mu.m and the flat-to-flat width of the whole fibre is around 40 .mu.m, with a hole spacing of around 2-3 .mu.m. If the diameter of the air holes in the fibre is a sufficiently small fraction of the pitch or spacing between the holes, the core of the fibre guides light in a single mode.
Single mode fibres have advantages over multi mode fibres in the field of long distance telecommunication, laser power delivery and many sensor applications due to the fact that a light signal carried by the fibre travels in only one mode and therefore avoids the problem of intermodal dispersion that is encountered with multi mode fibres. Also, at a given wavelength the intensity of light across a single mode fibre is guaranteed to follow a single smooth, known and unchanging distribution. This is regardless of how light is launched into the fibre or of any disturbance of the fibre (e.g. time varying).
In many applications it is advantageous for an optical fibre to carry as much optical power as possible as for example, any fibre inevitably attenuates the light passing through it. For example, for a given detector sensitivity, the length of a communications link can be increased by increasing the radiation power input to the fibre. As another example, there are many high power laser systems in industrial applications which could be made more simply if light could be channelled via a fibre rather than using conventional optical systems. There are, however, limits to the amount of light that can be carried by known optical fibres at a given time.
In a conventional fibre, comprising a core region surrounded by a lower refractive index cladding region, the material from which the fibre is made will ultimately suffer irreversible damage if the light intensity within the fibre exceeds a threshold value. At lower intensities, a number of intensity dependent non-linear optical processes can occur which, although non destructive to the fibre, nevertheless can degrade or even destroy an optical signal.
These problems may be alleviated by increasing the size of the core of the fibre which, for a given power, reduces the intensity of the light in the fibre, therefore allowing a greater power to be carried before the threshold for non linear processes are reached. However, if the core diameter alone is increased the fibre will become multi mode. This may be compensated by reducing the index difference between the core and the cladding. Eventually however, it becomes difficult to control the uniformity of doping across the core. Furthermore, fibres with small index differences are susceptible to loss of light at bends. Therefore, there are limits to the extent to which an increased core size can be used to increase the power capacity of a single mode fibre.
Some of the non-linear effects are exacerbated by the presence of dopants in the core, which make the material more susceptible to these effects. At higher powers, doped fibres are more susceptible to irreversible damage. Dopants also make the fibre more susceptible to damage by ionising radiation which is an issue in the nuclear industry. This has been combated by making the core out of pure silica. Total internal reflection is maintained by introducing dopants to the cladding which reduce its refractive index and as less light is carried in the cladding than in the core, more power can be carried. However, this is limited by the fact that some of the light is carried in the doped cladding.
Furthermore, in conventional fibres, efficient coupling of high power lasers into the fibre is problematic as the light needs to be focused into a small spot and the intensity at the endface of the fibre is therefore larger than it would be if the core were larger. Optical damage at or near the endface of the fibre frequently limits the power of radiation that can be launched into it [S. W. Allison et al., Appl. Opt. 24 (1985) p. 3140]. The maximum continuous wave (cw) power that has been achieved in a conventional single-mode fibre is only around 15 W.